1. Field of the Invention
The present invention generally relates to regenerative relay methods and regenerative relay apparatuses, and more specifically, to a regenerative relay method and regenerative relay apparatus whereby, for example, a regenerative relay of a signal is implemented in an optical transmission system such as a Dense Wavelength Division Multiplexer (DWDM) system.
2. Description of the Related Art
Recently and continuing, due to the increase of communications capability, a large amount of data can be transmitted by a transmission apparatus and long distance transmission of data can be done. In this state, an efficient optical transmission system using Wavelength Division Multiplexing is utilized. In this Wavelength Division Multiplex transmission, deterioration of light often happens due to mixing of noise by the optical amplifier, light dispersion or polarization by a long distance transmission, or the like. Because of this, instead of use of an expensive compensator or low noise amplifier, an error correction function whereby a signal is encoded by adding a redundancy bit and the signal is decoded at a receiving side, has been used.
An S/N ratio improved by the error correction is generally called a coding gain. Various kinds of error correction have been developed depending on their coding rules or the number of the redundancy bits. Generally, as the coding gain is larger, that is, the error correction ability is higher, the number of the redundancy bits is bigger, circuit size is bigger, delay time is longer, and consumption of electric power is larger.
Transmission capacity has been recently extremely improved so that a transmission capacity of 10 Gbit/s or 40 Gbit/s has been utilized. Therefore, an error correction method wherein the number of the redundancy bits, the circuit size, the delay time, and the consumption of electric power are respectively small but the coding gain is large in demanded.
In addition, an INVERSE-MUX method has been used. According to this method, a signal having a larger capacity is divided into plural routes and transmitted so that the signal is multiplexed at a transmitted side and the original signal is regenerated. In this method, for example, in order to divide and transmit a signal having a capacity of 40 Gbit/s into four routes of 10 Gbit/s each and then multiplex to generate the signal having a capacity of 40 Gbit/s again, it is necessary to minimize the differences of the delay times of the transmitted four routes and therefore the delay time is increased due to the number of the relay apparatuses of the respective routes.
FIG. 1 is a block diagram of an example of a related art transmission system. Referring to FIG. 1, in a terminal apparatus 10, a redundancy bit is added to a client signal by using an encoding part 12 so that the client signal is encoded. The coded signal is converted to an optical signal by an electric/optical conversion part 14. The converted optical signal is multiplexed by an optical wavelength multiplexer 16 so as to be sent out to a transmission path at a network side.
The optical multiplex signal that is sent out is optical-level generated by an optical amplifier (ILA) 171, 172, and 173 arranged on the transmission path of the network so as to be transmitted. In addition, the signals are divided into individual optical signals by an optical wavelength divider 18 so as to be transmitted to a regenerative relay apparatus 20.
In the regenerative relay apparatus 20, the signal converted to the electric signal by the optical/electric conversion part 22 is decoded so that the error correction is made and the signal is regenerated by the signal generation part 26. After that, a redundancy bit is added again by an encoding part 28 so that the signal is encoded. The signal is converted to an optical signal by an electric/optical conversion part 30, and then the signal is multiplexed by an optical wavelength multiplexer 32 so as to be sent out to the transmission path at the network side. The signal is transmitted to a terminal apparatus 34 situated in a remote position by repeating this process. In the terminal apparatus receiving the signal, after the error correction is made, the signal is used as a transmission signal to the client.
In the meantime, Japan Laid-Open Patent Application Publication No. 2000-341344 discloses that a signal converted to an electric signal by a light receiving element is binarized and demultiplexed into n-channels by a comparator, errors of the binarized signals of respective channels are corrected by an error correction circuit, a total error number obtained by counting the number of the errors of the channels is supplied to a threshold value control circuit, the threshold control circuit allows a threshold generation circuit to generate plural thresholds used by the comparator, and an optimum threshold is determined on the basis of the total error number.
In the related art transmission system, decoders having constant error correction abilities regardless of the transmission path qualities are provided in the regenerative relay apparatus and the terminal apparatus. Therefore, the signal passes through the decoder even at a section where the number of errors is small so that signal delay by the decoder and the encoder is increased.